Kawasaki Disease (KD) is the leading cause of acquired heart disease in children in developed nations. At Ann & Robert H. Lurie Children's Hospital of Chicago, we diagnose 60-70 new cases/year, and most US children's hospitals diagnose scores of KD patients annually. KD is an acute febrile illness with clinical features commonly observed in infectious diseases, can result in coronary artery aneurysms that can be particularly severe in infants <6 months of age, has highest incidence in young children <5 years of age, is extremely rare in adults, occurs in epidemics with geographic wavelike spread of illness, and only rarely recurs. These specific epidemiologic features of KD are best explained by infection with a ubiquitous infectious agent that is usually asymptomatic, but can result in KD in genetically susceptible children. No known infectious agent is associated with KD. My laboratory reported an antigen-driven arterial IgA immune response in acute KD. In prior studies, we made synthetic antibodies using oligoclonal immunoglobulin ? heavy chains and random light chains from KD arterial tissue cDNA. Cognate light and heavy chain pairs could not be determined from the tissue cDNA available to us. Our original KD synthetic antibodies detected antigen in intracytoplasmic inclusion bodies in acute KD but not infant control bronchial epithelium by immunohistochemistry, but were not of sufficient affinity to bind antigen in immunoprecipitation assays. In other studies, we found that type I interferon-induced genes were upregulated in lungs and coronary arteries of KD patients, consistent with an antiviral immune response. Based on these data, we hypothesize that KD is an immune-mediated illness occurring in genetically predisposed children infected with a presently unidentified virus. In this proposal we will use new methods to rapidly clone and produce antigen-specific synthetic antibodies from single cell sorted peripheral blood plasmablasts, allowing identification of cognate light and heavy chains in individual plasmablasts and production of high-affinity antibodies. We plan to prepare a panel of KD-specific monoclonal antibodies and use them to identify KD-specific antigens. In preliminary studies, we identified an oligoclonal, antigen-driven plasmablast response in peripheral blood of an acute KD patient who developed a coronary artery aneurysm. We prepared synthetic antibody using cognate light and heavy chains from the most prevalent oligoclonal plasmablasts from this patient. This antibody binds to antigen in tissues from other KD patients. This proposal includes a multidisciplinary team of experts in KD, virology, immunology, and biostatistics to perform the following specific aims: 1) Identify and clone the peripheral blood plasmablast response in KD, and 2) Identify antigenic targets of oligoclonal KD peripheral blood plasmablasts. Identification of KD-specific antibodies and antigens is an important step toward identifying KD etiology, improving diagnosis and treatment, and enabling prevention, with the goals of reducing pediatric health care costs, morbidity, and mortality from the long-term consequences of coronary artery aneurysms that are acquired during young childhood.